Exposure to audio signals at greater and greater amplitudes through the use of headphones and media devices, such as cell phones and MP3 players, has been increasing at an alarming rate. Exposure to audio signals at high decibel levels has been determined to be one of the primary causes of age-related permanent hearing impairment. However, hearing impairment is not only increasing in the general population, but is increasing at a significantly faster rate among young people, especially in among those who utilize media devices and wear headphones (or wireless earpieces) for significant amounts of time.
The extent of hearing damage sustained through exposure to sounds has been determined to be a function of both the amplitude and the duration of the audio signals, and particularly exposure to audio signals at amplitudes that exceed a safe acoustic threshold. Permanent hearing damage is a cumulative effect of exceeding the minimum thresholds or safe pressure levels for extended periods. Safe listening durations at various amplitudes can be calculated by averaging audio output levels over time to yield a time-weighted average. Various administrative bodies (such as the Occupational Safety and Health Administration (OSHA)) and health awareness agencies (such as the National Institute for Occupational Safety and Health (NIOSH)) have adopted guidelines for safe acoustic levels that are based on an eight hour work day. However, such guidelines were not necessarily designed to address the most common source of acoustic damage, namely headphones.
Unfortunately, most common media devices and their associated headphones encourage listening to music at volume levels well above the safe acoustic threshold set, for example, by OSHA. Such volume levels may have no immediate effect on hearing, but long-term exposure can nevertheless cause permanent hearing impairment.
To help prevent hearing damage, some devices have been developed to periodically measure sound levels of ambient audio signals. Such measurements can be used to estimate a cumulative effect of the ambient audio signals over time. However, such devices often simply notify the user when they have exceeded the OSHA or NIOSH guidelines for acoustic exposure. Unfortunately, these devices typically provide no preventative measures for the device user. Further, such devices are often worn in place of headphones, making the two devices incompatible. Some headphones utilize a predetermined maximum output level in an attempt to limit the output amplitude to prevent ear damage. This approach, however, is ineffective as it does not take into account listening duration and the calculation of risk for auditory injury over time.
Other devices have been developed to be placed as an accessory between the media player and the earphones increasing earphone impedance as the decibel level increases. This approach, however, is limited, in part, because such devices cannot be calibrated for the speakers in the headphones. As a result, these devices may either limit the audio output too much or not enough.
In the following description, the use of the same reference numerals in different drawings indicates similar or identical items.